As is known, circular knitting machines are equipped with one or more sets of yarn guides which are designed to dispense one or more yarns to the needles of the machine and are arranged in a circular bed. The yarn guides and the bed of the machine move with respect to each other, so that the yarn guides can dispense the yarns to the various needles arranged side-by-side inside the bed.
In some kinds of machines, the bed is fixed while the yarn guides are mounted on a supporting element which is actuated with a rotary motion about the axis of the bed with respect to the bed. In this case, the needle actuation cams also are actuated, together with the yarn guide supporting element, with a rotary motion about the axis of the bed so as to cause the movement of the needles which forms the knitting.
In other types of machines, the yarn guide supporting element and the needle actuation cams are fixed and the bed is actuated with a rotary motion about its own axis.
Each yarn guide has, proximate to one of its ends directed toward the bed of the machine, a passage for the yarn to be dispensed, which arrives from a spool or from a yarn magazine.
In order to change the yarns delivered to the needles during knitting, for example to form patterns, each yarn guide can move on command from an inactive position, in which it is arranged so that its passage for the yarn lies on the inner side of the bed, i.e., between the bed and the axis of the bed, to an active position, in which it is arranged so that its passage for the yarn lies on the outer side of the bed, and vice versa. In this manner, when the yarn guide is in the active position, the yarn dispensed by the yarn guide straddles the bed and its engagement by the needles that are moved to knit is ensured, while when the yarn guide is in the inactive position the passage for the yarn is arranged inside the bed, preventing the engagement of the yarn by the needles.
These devices are generally equipped, for each yarn guide, with an element for clamping and cutting the yarn dispensed by the corresponding yarn guide. Such clamping and cutting element is designed to retain the yarn by clamping while the corresponding yarn guide is in the inactive position, to release the yarn when the corresponding yarn guide is moved into the active position, and to engage the yarn again and cut it when the corresponding yarn guide is returned to the inactive position. To do this, each clamping and cutting element can move toward and away from the bed in order to pass from a position for clamping the yarn to position for waiting for the yarn to be clamped and vice versa.
In known types of devices there is generally a single actuator to produce the passage of the yarn guides from the inactive position to the active position, and each yarn guide can be moved on command, by means of a corresponding individual actuation element, from a deactivation position, in which it is excluded from the action of this actuator, to an activation position, in which it can undergo the action of the actuator.
Usually, the actuator that acts on the yarn guides that are in the activation position is arranged in a specific region of the bed which is dedicated specifically to the operation for changing the yarn guides that are being used and is constituted generally by a cam. Likewise, in the same region of the bed there is generally another actuator, usually another cam, which produces the passage of the yarn guides that are in the active position from the active position to the inactive position.
In known types of devices, due to the fact that the actuators that actuate the passage of the yarn guides from the inactive position to the active position are arranged at a preset region of the bed, it is not possible to change the yarn guides that are active except at such region and therefore it is not possible to limit the dispensing of a yarn to a region of a formed row of knitting, but the yarn must be dispensed for an entire row of knitting. This fact constitutes a limitation of the kinds of knitting that can be performed with machines equipped with these devices.